1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer which uses an electrophotographic system or an electrostatic recording system.
2. Related Background Art
FIG. 11 shows an image forming apparatus of the electrophotographic system.
In the figure, the image forming apparatus includes a drum-shaped electrophotographic photosensitive member (hereinafter referred to as xe2x80x9cphotosensitive drumxe2x80x9d) 1 which rotates in a direction indicated by an arrow a (clockwise). A charging roller 2, an exposing apparatus 3, a developing apparatus 4, a transfer roller 5 and a cleaning apparatus 6 are disposed around the photosensitive drum 1.
Upstream of a transferring nip portion N between the photosensitive drum 1 and the transfer roller 5 in a conveying direction of a transferring material P, a sheet feeding cassette 7, a sheet feeding roller 8, a pre-feed sensor 9, a pair of resist rollers 10, a top sensor 11 and a transfer guide 12 are disposed. On the other hand, downstream of the transferring nip portion N in the conveying direction of the transferring material P, an electricity erasing needle 13, a conveying guide 14, a fixing apparatus 15 and a pair of sheet discharging rollers 16 are disposed.
Also, the image forming apparatus can perform duplex image formation (duplex printing), and a pair of reverse rollers 18, a pair of sheet re-feeding rollers 19 and a sheet re-feeding sensor 20 for duplex image formation are disposed in a duplex conveying path 17.
The photosensitive drum 1 is formed of a negatively charged OPC photosensitive member which is rotationally driven in the direction indicated by the arrow a at a given process speed in a conventional example.
The charging roller 2 is brought in contact with a surface of the photosensitive drum 1 by a predetermined pressing force and charges the photosensitive drum 1 to a predetermind polarity and potential by a charging bias applied from a charging bias power supply (not shown).
The exposing apparatus 3 includes a laser diode that emits a laser beam, a collimator lens, a polygon mirror, an fxcex8 lens, etc., not shown, and scans the photosensitive drum 1, which has been uniformly charged by the charging roller 2, with an outgoing laser beam L, which has been on/off-controlled in accordance with inputted image information, in a direction perpendicular to the rotating direction of the photosensitive drum 1 and then exposes light. The exposure allows charges on a portion of the photosensitive drum 1 which has been scanned with the laser beam L to be removed, and an electrostatic latent image to be formed on the surface of the photosensitive drum 1.
The developing apparatus 4 includes a rotatable developing sleeve 4a inside of which a magnet roller (not shown) is fixed. The developing apparatus 4 coats a developer (toner) on a developing sleeve 4a into a thin layer and attaches the toner onto the electrostatic latent image formed on the surface of the photosensitive drum 1 at a developing position to develop (visualize) the electrostatic latent image as a toner image. The developing sleeve 4a is applied with a developing bias from a developing bias power supply (not shown).
The transfer roller 5 is brought in contact with the surface of the photosensitive drum 1 by a predetermied pressing force to form the transferring nip portion N, and transfers the toner image on the surface of the photosensitive drum 1 onto the transferring material P at the transferring nip portion N defined between the photosensitive drum 1 and the transfer roller 5. The transfer roller 5 is rotated in a direction indicated by an arrow b (counterclockwise).
The fixing apparatus 15 includes a fixing roller 15a and a pressure roller 15b. The fixing apparatus 15 heats and pressurizes the transferring material P onto which the toner image has been transferred between the fixing roller 15a and the pressure roller 15b to thermally fix the toner image onto the surface of the transferring material P.
Subsequently, the image forming operation of the above-described image forming apparatus will be described.
Upon the issuance of an image forming operation start signal, the transferring materials P within the cassette 7 are fed one by one by the sheet feeding roller 8 and conveyed up to the pair of resist rollers 10. During this operation, the conveyance of the transferring materials P is detected by the pre-feed sensor 9.
On the other hand, the photosensitive drum 1 is rotationally driven in the direction indicated by the arrow a by driving means (not shown) and charged to the predetermined potential by the charging roller 2. Then, the laser beam L corresponding to an image signal is irradiated onto the photosensitive drum 1 from the exposing apparatus 3, and the potential on the photosensitive drum 1 at a portion onto which the laser beam L has been irradiated drops to form the electrostatic latent image. Then, the toner is attached onto the electrostatic latent image formed on the photosensitive drum 1 by the developing sleeve 4a of the developing apparatus 4 to which the developing bias having the same polarity as the charging polarity (negative polarity) of the photosensitive drum 1 is applied, to thereby visualize the electrostatic latent image as the toner image.
Then, after a leading end of the transferring material P is detected by the top sensor 11, the transferring material P is fed by the pair of resist rollers 10 to the transferring nip portion N between the photosensitive drum 1 and the transfer roller 5 through the transfer guide 12 in synchronism with the rotation of the photosensitive drum 1. Then, the transfer roller 5 is applied with a transferring bias having the reverse polarity (positive polarity) of the toner from the transferring bias power supply (not shown), and the toner image is transferred onto the transferring material P from the photosensitive drum 1.
Then, the transferring material P onto which the toner image has been transferred is electricity-erased by the electricity erasing needle 13 to which charges having the reverse polarity (negative polarity) of the transfer roller 5 are given, and the transferring material P is separated from the photosensitive drum 1 by the own weight of the transferring material P. The transferring material P which has been separated from the photosensitive drum 1 is conveyed to the fixing apparatus 15 through the conveying guide 14, and the transferred toner image is thermally fixed onto the surface of the transferring material P due to heating and pressurizing by the fixing roller 15a and the pressure roller 12b of the fixing apparatus 15. Then, the transferring material P is discharged to the external from the pair of sheet discharging rollers 16.
On the other hand, a residual toner (untransferred toner) and other attachments are removed from the surface of the photosensitive drum 1 from which the toner image has been transferred and cleaned by the cleaning apparatus 6, and the photosensitive drum 1 is put to succeeding image formation.
Also, at the time of duplex image formation (double-sided print), the transferring material P which has been discharged from the fixing apparatus 15 after the image has been formed on a first surface (front surface) of the transferring material P is reversed by the reverse rollers 18 and conveyed to the duplex conveying path 17. Then, the transferring material P is allowed to pass through the sheet re-feeding sensor 20 by the sheet re-feeding rollers 19 and again introduced to the pair of resist rollers 10. Thereafter, another image is formed on a second surface (back surface) of the transferring material P in the same manner as the first surface.
Note that at the time of the above-described image formation, the transfer roller 5 is applied with a transferring bias Vtr required for transferring the toner image from the transferring bias power supply (not shown) to give charges to the transferring material P that passes through the transferring nip portion N between the photosensitive drum 1 and the transfer roller 5 so that the toner image is transferred onto the transferring material P from the photosensitive drum 1. After the toner image has been transferred onto the transferring material P, a voltage applied to the transfer roller 5 is turned off or changed to a low bias Vlow lower than the transferring bias Vtr, to thereby prevent a drum memory or a sheet mark from occurring at the time of non-transferring.
Assuming that an image forming area (printable area) S of the transferring material P is, as shown in FIG. 12, b mm (for example, 5 mm) inside of a leading end P1, a rear end P2, and right and left ends P3, respectively, the changing of the bias from the transferring bias Vtr after transferring to the low bias Vlow is conducted, as shown in FIG. 13, in a non-image area Sxe2x80x2 inside of the rear end P2 of the transferring material P by a distance c (c less than b) mm. In this example, the conveying direction of the transferring material P is a direction indicated by an arrow A.
As described above, in the case of changing the transferring bias, a timing of changing from the transferring bias to the low bias at the rear end P2 of the transferring material P is decided by reckoning a time at which the rear end P2 of the transferring material P reaches the transferring nip portion N from a time at which the rear end P2 of the transferring material P passes through the top sensor 11 upstream of the transferring nip portion N, a distance between the top sensor 11 and the transferring nip portion N, and a conveying speed of the transferring material P backward.
However, the distance between the top sensor 11 and the transferring nip portion N is slightly varied by the respective manufactured image forming apparatuses, and the conveying speed of the transferring material P also depends on the outer diameter of the transfer roller 5, the sort of the transferring material P, the printing ratio, etc., and slightly varies. For that reason, there arises such a problem that a timing of changing from the transferring bias to the low transferring bias is slightly shifted.
Also, the top sensor 11 includes, as shown in FIG. 14, a sensor lever 21 movable by contact with the transferring material P that passes through the sensor lever 21, and a photo-interrupter 22 that detects the moved sensor lever 21 in a non-contact manner, and detects the leading end and the rear end of the transferring material P that passes through the top sensor 11.
However, in the top sensor 11, since the movable range of the sensor lever 21 depends on which portion of the sensor lever 21 the transferring material P passes through, there arises such a problem that an error occurs in a period of time until the photo-interrupter 22 detects the sensor lever 21, to thereby shift the timing of changing the transferring bias.
As described above, if the timing of changing the transferring bias is shifted, for example, in the case where the changing timing is shifted toward the rear end side of the transferring material P, an excessive transferring current flows in the photosensitive drum 1 side at an edge portion of the rear end of the transferring material P. For that reason, the potential fluctuates due to the excessive current that flows at the rear end of the transferring material P on the photosensitive drum 1, as a result of which there occurs a phenomenon that the potential fluctuation appears as a memory or a stripe on the photosensitive drum 1 at the time of printing a next transferring material P (hereinafter referred to as xe2x80x9crear end memoryxe2x80x9d).
On the contrary, in the case where the timing of changing the transferring bias is shifted toward the inside from the rear end of the transferring material P, the timing of changing the transferring bias is applied to the image forming area, and for example, at the time of printing the second surface of the transferring material P (a rear-surface printing at the time of the duplex image formation) or printing a high-resistant transferring material (paper) P, the transferring current is short when the bias is changed to the low transferring bias, thereby leading to such a problem that the toner image scatters.
In particular, the timing of changing the transferring bias is more shifted as the process speed becomes higher, and the high-speed printing operation of the image forming apparatus in recent years makes it difficult to provide latitude to that shift in the non-image area at the rear end of the transferring material P.
The present invention has been made under the above circumstances, and therefore an object of the present invention is to provide an image forming apparatus which does not allow any memory corresponding to a rear end of a transferring material to occur.
Another object of the present invention is to provide an image forming apparatus which does not allow a toner image on the rear end of the transferring material to scatter.
Still another object of the present invention is to provide an image forming apparatus which is capable of changing a transferring bias at an optimum timing.
Yet still another object of the present invention is to provide an image forming apparatus, comprising:
an image bearing member which bears a toner image;
a transferring member which forms the image bearing member and a nip and transfers the toner image onto a transferring material;
bias applying means for applying a transferring bias to the transferring member, the bias applying means changing from the transferring bias to a low bias lower than the transferring bias at the rear end portion of the transferring material; and
control means for controlling a position at which the transferring bias is changed to the low bias.
Yet still other objects of the present invention will become apparent from the following description.
These and other objects, features and advantages of this invention will become more fully apparent from the following detailed description taken with the accompanying drawings.